This invention relates generally to fuel cells and more specifically to compliant fuel cell systems designed to accommodate strain in the fuel cell assemblies during thermal cycles.
A fuel cell produces electricity by catalyzing fuel and oxidant into ionized atomic hydrogen and oxygen at the anode and the cathode, respectively. The electrons removed from hydrogen in the ionization process at the anode are conducted to the cathode where they ionize the oxygen. In the case of a solid oxide fuel cell, the oxygen ions are conducted through the electrolyte where they combine with ionized hydrogen to form water as a waste product and complete the process. The electrolyte is otherwise impermeable to both fuel and oxidant and merely conducts oxygen ions. This series of electrochemical reactions is the sole means of generating electric power within the fuel cell. It is therefore desirable to reduce or eliminate any mixing of the reactants that results in a different combination such as combustion which does not produce electric power and therefore reduces the efficiency of the fuel cell.
The fuel cells are typically assembled in electrical series in a fuel cell stack to produce power at useful voltages. To create a fuel cell stack, an interconnecting member is used to connect the adjacent fuel cells together in electrical series. When the fuel cells are operated at high temperatures, such as between approximately 600° C. (Celsius) and 1000° C., the fuel cells are subjected to mechanical and thermal loads that may create strain and resulting stress in the fuel cell stack. Typically in a fuel cell assembly, various elements in intimate contact with each other comprise different materials of construction, such as a metal and a ceramic. During the thermal cycles of the fuel cell assembly, elements expand and/or contract in different ways due to the difference in the coefficient of thermal expansion (CTE) of the materials of construction. In addition, individual elements may undergo expansion or contraction due to other phenomena, such as a change in the chemical state of one or more elements. This difference in dimensional expansion and/or contraction may affect the seal separating the oxidant and the fuel paths and also the bonding of the elements made of dissimilar materials.
Therefore there is a need to design a fuel cell assembly that is compliant to changes in operating states including temperature cycles and changes in chemical state. Furthermore to preserve the mechanical integrity of the fuel cell assembly, the compliant fuel cell assembly needs to be designed in such a way that any expansion in the fuel cell assembly at high temperatures does not create stress in the fuel cell assembly.